Pheochromocytomas are uncommon tumors that are hard to diagnose, but exceedingly dangerous. to facilitate diagnosis, we have recently developed a new technique for measuring plasma concentrations of free metanephrines, using HPLC and electrochemical detection. Use of this test in 52 patients with pheochromocytoma, and 191 normals or patients with other disorders, indicated that the sensitivity of plasma metanephrines was 100% and the negative predictive value of normal plasma concentrations of metanephrine was also 100%. Thus, normal plasma concentrations of metanephrines exclude the dagnosis of pheochromocytoma, whereas normal values for two commonly used tests for pheochromocytoma, i.e., plasma catecholamines and urinary metanephrines, do not. Preliminary results from data on patients with pheochromocytoma as part of familial von Hippel-Lindau disease indicate that the pheochromocytoma in such affected kindreds are frequently asymptomatic and/or non functioning. In addition, small early VHL pheochromocytomas appear to be biochemically inactive by standard testing and become more functional with time. This indicates that VHL kindreds, particularly those with a history of pheochromocytoma, must be carefully and repeatedly screened in order to prevent potential morbidity and mortality from an undiagnosed pheochromocytoma.